There are several processes such as printing, offset lithography and rotary die cutting in which a web of indeterminate length, drawn off of a supply roll, is moved lengthwise forward through an impression zone at which it passes through a nip defined by a delineating cylinder that contacts one surface of the web and an opposing cylinder that engages its opposite surface. The delineating cylinder, which carries means for transferring ink or the like to the web, or for cutting or scoring the web, rotates in step with forward movement of the web to make uniform impressions upon it at regular repeat intervals along its length.
In many cases, apparatus for performing these processes should be adjustable for changing the length of the repeat interval, to be capable of running different jobs. An expedient for accomplishing such adjustability is broadly disclosed in each of U.S. Pat. No. 2,758,541 to R. A. Tison (1956), No. 2,845,021 to Pinckert et al (1958), and No. 3,756,149 to T. D. Bishop (1973). In these, the delineating cylinder has a cutout or reduced radius peripheral portion that extends around a part of its circumference, to be out of contact with the web through the fraction of each revolution during which the cutout is juxtaposed to the opposing cylinder. This fraction of a revolution can be considered an idle phase in the rotational cycle of the delineating cylinder. Through an advancing phase which constitutes the remainder of its cycle and during which the delineating cylinder can be in contact with the web, the forward speed of the web is maintained equal to the peripheral speed of that cylinder and the web moves forward though an invariable advancing distance. During the idle phase the web is moved rearward through a decremental distance which is less than the advancing distance. The difference between those distances is the repeat interval length, which can be changed if the decremental distance can be changed.
In the apparatus of the Tison patent, the web is trained in a loop around each of a pair of control rollers, one upstream from the delineating cylinder nip and the other downstream from it. Those control rollers are in turn carried by a frame which reciprocates forwardly and rearwardly to increase and decrease the length of web in each loop. The frame moves forward during the advancing phase of the delineating cylinder cycle, increasing the loop around the downstream control roller and permitting the loop upstream from the nip to decrease in size, thus moving the web through the advancing distance. Rearward movement of the frame during the idle phase effects an opposite change in the sizes of the loops and thus tends to move the web through a decremental distance equal to the advancing distance. However, during rearward movement of the frame, intermittently driven feed rollers downstream from the frame draw the web forward through a distance equal to the repeat interval length. One of the feed rollers carries a coaxial pinion, to be rotatably driven by endwise movement of a ratchet-toothed rack; and the rack, in turn, has one end connected to a rotating eccentric to be reciprocated lengthwise by rotation of the eccentric, which is synchronized with rotation of the delineating cylinder and reciprocation of the frame. The feed rollers are apparently rotated only during rearward movement of the frame but maintain their grip on the web, without rotation, during forward movement of the frame.
With the apparatus as disclosed by Tison, the web is drawn off of the supply roll intermittently, and therefore the rotational inertia of a heavy supply roll will pose serious problems. A more important deficiency is that the frame that carries the control rollers moves through the same distances and at the same speeds in its forward and its rearward strokes, and therefore the reduced radius portion of the delineating cylinder must extend around at least half of its circumference. This is to say that the delineating cylinder is effectively idle during at least half of each cycle, and output is correspondingly low. Since the frame that carries the control rollers is necessarily rather massive, its forward and rearward strokes must be made at relatively low speed to avoid inertia problems, further contributing to a low rate of production. The manner in which the feed rollers are driven for intermittent rotation also appears to be unsuitable for high speed operation.
The delineating cylinder of the Pinckert et al patent has a radial cutout which extends around only a small fraction of its circumference. During the short idle phase in which that cutout is juxtaposed to the opposing cylinder, the web is drawn rearward by a pair of feed rollers located a small distance upstream from the delineating cylinder. The feed rollers constantly rotate in the direction opposite to that of delineating cylinder rotation and in synchronism with that cylinder, but one of the feed rollers has a reduced radius around a major portion of its circumference, so that it cooperates with the other to grip the web and draw it rearward only during the idle phase of the delineating cylinder cycle. A dancer arrangement upstream from the feed rollers takes up and pays off web as necessary to maintain a steady rate of withdrawal of web from the supply roll.
With this Pinckert et al arrangement the web is, in theory, either being drawn forwardly by the operative peripheral portion of the delineating cylinder or drawn rearwardly by the operative peripheral portion of the one feed roller. However, the web cannot reverse its direction of lengthwise movement instantaneously, and the inevitable decelerations and accelerations must be accommodated by slippages which are indeterminate and variable, so that repeat interval length cannot be accurately maintained. Another and very important deficiency of this prior apparatus is that it has no provision for adjustably varying the decremental distance through which the web is moved rearwardly by the feed rollers, and therefore it does not provide for adjustable variation of repeat interval length.
In the apparatus disclosed by Bishop, the web is moved forward during the advancing phase by its engagement with the delineating cylinder. Accelerations, decelerations and rearward movements of the web during the idle phase are effected by means of a pair of feed rollers located just upstream from the nip that comprises the delineating cylinder, one rotatably driven and the other freely rotatable. During the advancing phase the freely-rotatable feed roller is spaced from the driven one; during the remainder of the cycle the feed roller nip is closed and a pinion coaxially fixed on the shaft of the driven feed roller is rotated by means of an arcuate rack which swings about a fixed axis spaced from the feed roller shafts. The arcuate rack is actuated for swinging back and forth motion, to impart forward and reverse rotations to the driven feed roller, by means of a pair of cams mounted on one end of the delineating cylinder and circumferentially adjustable relative to it, engaged by a cam follower on the rack. The arrangement is such that the delineating cylinder can produce an impression through only a little over half of each cycle, and therefore the rate of production tends to be slow. Accurate circumferential adjustment of the cams on the delineating cylinder, for adjustment of the decremental distance, seems to be difficult, at least in the form of the apparatus that the patent discloses. A further disadvantage for many applications is that during the advancing phase the web is driven only by the delineating cylinder, and therefore the apparatus is suitable only for rotary die cutting operations, for which it is specifically intended. In printing or offset work there would be a possibility of slippage of the delineating cylinder relative to the web.
In view of the deficiencies of the mechanisms disclosed in these prior patents, it is understandable that none of them is known to have had any significant commercialization. Instead, variation in repeat interval length has heretofore been provided for by means of interchangeable parts. A delineating cylinder had to be provided for each of a number of stepwise differing repeat interval lengths, each delineating cylinder having a circumference matched to the repeat interval for which it was intended, and for each such cylinder there had to be a special set of gears by which rotation of the delineating cylinder was synchronized with rotation of the several other cylinders and rollers on the machine.
With interchangeable delineating cylinders and their respective gear sets, a large capital investment was needed to equip a machine for a full range of repeat interval lengths, and some of that investment was always idle. Interchange of parts to convert from one repeat interval length to another was time consuming, and the cost of the skilled labor needed for such interchange was compounded by the loss of production while the interchange was being accomplished. A further important disadvantage was that only stepwise differing repeat interval lengths were available. Where the repeat interval length specified for a particular job fell between those respectively provided for by two sets of interchangeable parts, the job had to be run with the larger repeat interval, leaving the web with blanks between successive impressions along its length. In subsequent cutting of the web to the specified repeat interval lengths, these blank portions had to be discarded, increasing the cost of the job by the cost of the scrapped web material. Such web wastage tended to be especially high with apparatus that made no provision for varying the repeat interval length, as pointed out in the above-discussed Bishop patent, which also mentions that in the production of carton blanks and the like such wastage was often avoided by cutting the web into discrete lengths which were individually fed through the appartus, "thus introducing an additional machine and an extra handling step."